1. Field of the Invention
The invention relates to a force sensor or a force sensor unit, in particular for an elongate device, and a method for measuring a force and its effective direction using the force sensor.
2. Description of Related Art
A particular application of this invention relates to catheter technology, determined by an elongate device for at least partial insertion into an organism through a body orifice. These elongate devices are used in particular in minimally invasive surgery and in the diagnosis of in particular human bodies. To ensure that, during the invasion by the elongate device, no body vessels are injured by the body-end tip of the elongate device which is usually operated manually by the treating medical practitioner, the medical practitioner necessarily orients himself according to the forces which are relayed to him by the handle of the elongate device. Due to friction and the inserted mass of the catheter or guide wire, which continuously increases during the course of the invasion of the catheter or guide wire in the body, the force relayed to the treating medical practitioner by the handle hardly provides any useful information regarding the forces which are actually occurring at the tip of the catheter or guide wire. So that the treating medical practitioner can impart the correct operating force on the handle of the catheter, extraordinary experience of operating catheters or guide wires is necessary.
DE 103 03 270 A1 discloses a catheter arrangement in which the force applied to the catheter tip during the insertion is measured. The corresponding force is relayed to the medical practitioner in a tactile fashion by means of a haptic handle. In particular for an inexperienced medical practitioner, this simplifies finding artery branches or perforations on the cardiac septum for example, and forms the basis of intuitive handling of the device. An electrodynamic drive apparatus using a measurement variable, representing the force at the tip, to generate haptic force pre-stressing is disclosed by DE 103 19 081 A1. The invention presented here makes it possible to measure the force at the tip of the elongate device and to evaluate the measurement signals, which is required for implementing the invention from DE 103 03 270 A1.
In accordance with U.S. Pat. No. 6,221,023 B1, provision is made for a force sensor at the tip of catheters which is based on a resistive functional operation. The force induced in the sensor is recorded by a resistor bridge circuit. The design of this sensor is complex due to the large number of parts, and the production and assembly costs associated with this mean that the known sensor is unsuitable for catheters in particular due to the preferred single-use property of the latter. Furthermore, the surface for the primary contacting of the measurement element is designed to be perpendicular to the longitudinal direction of the catheter, and hence the surface for contacting is limited by the diameter of the elongate device. It is for this reason, and due to the large number of parts, that miniaturization possibilities of the force sensor, in particular below a catheter diameter of less than 3 mm, can only be implemented with a very high design complexity. Due to the large dimensions, it is possible to conclude that the sensor completely closes off the catheter tube at the tip. However, this means that the functionality of the catheter tube, by means of which instruments and liquids are inserted into the body, is no longer given. Integrating the sensor into the substantially thinner guide wire required during catheterizations is not possible due to the large dimensions, the large number of parts and the inexpedient wire guide through the contact surface oriented perpendicularly to the longitudinal direction. JP 06190050 A describes a tactile sensor which can be attached to the outer wall of catheters. It is proposed to produce this sensor from a very thin silicon disk. However, it is not possible for very thin elements to record the occurring forces in the region of approximately 300 mN. The use of thicker, more stable silicon disks can be discarded, since bending thicker disks, which is required for integration into the device, would lead to the material breaking.
“Beccai L et al.: Silicon-based three axial Force Sensor for Prosthetic Applications, Sensors and Microsystems, Proceedings of the 7th Italian Conference 2002” present a force sensor which is intended to be integrated into leg prostheses. The force vector can be determined by measuring the direction and magnitude of the force. Resistive elements, which are not interconnected to form a Wheatstone bridge, are used to measure the forces. However, bridge interconnections which can attain high measurement accuracy are sought after for the integration into an elongate device because in this case the signals can be transmitted almost independently of the feed line resistances in the long, thin feed lines. Otherwise the measurement signal depends on the temperature of the feed line, and hence on the insertion length of the device, particularly in the case of long devices inserted into the body. The production of the elements requires a two-sided, multistage dry etching process. Nevertheless, the measurement element must still be mounted over a special counter body to make a deflection of the measurement beams possible. The sensor thus comprises at least two parts which have to be connected accurately to one another. The exposed force detector, referred to as a mesa, is designed to have a long length; it approximately has a length which corresponds to the thickness of the original material to be precise. Hence this sensor always has, due to its operating principles, both a very much increased measurement sensitivity and susceptibility to destruction toward lateral forces compared to the force applied along the longitudinal direction. This is inexpedient because it is desirable in particular to measure the forces in the longitudinal direction of the device, or to obtain a balanced sensitivity for different force components. The measurement element has a plane of electrical contacts which is perpendicular to the longitudinal direction, and so the contacting surface on the diameter of the installation area is limited, which makes contacting more difficult. The measurement element has a diameter of approximately 1 mm, which is large compared to guide wires, so integration into the guide wire is impossible. This shows that the prior art relating to tactile sensors for integration into catheters and guide wires does not conform to the imposed requirements with respect to miniaturization, high stability, simple production and low costs.